1. Field of the Invention
The present invention relates to monolithic ceramic substrates, to manufacturing and designing methods therefor, and to electronic devices including such a monolithic ceramic substrate. In particular, the present invention relates to an improvement for reducing warpage of the monolithic ceramic substrate.
2. Description of the Related Art
Monolithic ceramic substrates include a plurality of ceramic layers laminated together to define a laminate body. In the monolithic ceramic substrate having the structure described above, various wiring conductors are provided. As wiring conductors, for example, internal conductive films extending along predetermined interfaces between ceramic layers and via hole conductors extending so as to penetrate predetermined ceramic layers are provided inside a monolithic ceramic substrate, and external conductive films are arranged to extend on the external surfaces of the monolithic ceramic substrate.
Monolithic ceramic substrates are used for mounting semiconductor chip units, other chip units, and other electronic components, and are used for interconnection of these electronic units. The wiring conductors described above define electric pathways for the interconnection described above.
In addition, passive units, such as capacitors, and inductors, may be embedded in monolithic ceramic substrates in some cases. In the case described above, these passive units are defined by parts of the internal conductive films and the via hole conductors used as the wiring conductors described above.
Monolithic ceramic substrates are used for, for example, LCR hybrid high frequency components in the field of terminal apparatuses for mobile communication. In addition, in the field of computers, monolithic ceramic substrates are used for forming hybrid components including active units, such as semiconductor integrated circuit (IC) chips, and passive units, such as capacitors, inductors, and resistors, or are used for merely forming semiconductor IC packages.
In particular, laminated ceramic electronic components are widely used for constituting various electronic devices, such as PA module substrates, RF diode switches, filters, chip antennas, various package devices, and hybrid devices.
In order to improve the multi-functionality, mounting densities, and performances of the monolithic ceramic substrates, it is effective to form wiring conductors having finer pattern densities.
However, in order to form a monolithic ceramic substrate, a sintering step must be performed. In the sintering step mentioned above, sintering of the ceramic causes shrinkage, and the shrinkage does not occur uniformly over the entire monolithic ceramic substrate, whereby undesired deformation and warping of the wiring conductors may be generated. The deformation and warping of the wiring conductors interfere with the improvement in wiring density of the wiring conductor.
Accordingly, a so-called non-shrinking process is proposed for use in manufacturing of monolithic ceramic substrates, in which the shrinkage of the monolithic ceramic substrate in the direction along the main surface can be substantially constrained during a sintering step.
In a method for manufacturing monolithic ceramic substrates in accordance with the non-shrinking process, in addition to a low-temperature sinterable ceramic material which can be sintered at, for example, 1,000xc2x0 C. or less, an inorganic particle is prepared which constrains the shrinkage and which is not sintered at a sintering temperature of the low-temperature sinterable material described above. When a green laminate is prepared which forms a predetermined monolithic ceramic substrate by sintering, constraining green layers containing the inorganic particle are disposed so as to be in contact with the main surfaces of predetermined layers of a plurality of base green layers which are laminated with each other and which contain the low-temperature sinterable ceramic material. In addition, conductive paste bodies for forming wiring conductors are provided for the base green layers.
The green laminate thus obtained is then fired. During this sintering step, reaction layers having a thickness of approximately about 2 xcexcm to about 3 xcexcm are formed at the interfaces between the base green layers and the constraining green layers, and the reaction layer adheres the base green layer to the constraining layer adjacent thereto. In addition, since the inorganic powder material contained in the constraining green layers is not substantially sintered, substantial shrinkage is unlikely to occur in the constraining green layers. Accordingly, since the constraining green layers constrain the shrinkage of the base green layers, the base green layers substantially shrink only in the thickness directions thereof, and the shrinkage in the directions along the main surfaces is constrained. As a result, since irregular deformation is difficult to occur in the monolithic ceramic substrate formed by sintering the green laminate, unwanted deformation and warping hardly occur, whereby a higher pattern densities of the wiring conductors can be achieved.
However, even though the shrinkage of the base green layer can be constrained in the direction along the main surface thereof, the shrinkage cannot be reduced to 0%, and since binders are lost which are contained in the base green layer and the constraining green layer, shrinkage of at least 2 to 3% inevitably occurs.
In addition, the shrinkage described above varies in accordance with the characteristics of the base green layer and the constraining green layer. For example, when the thickness of the base green layer is increased, it becomes difficult for the constraining force of the constraining green layer to work on the base green layers, and as a result, the base green layer is more likely to shrink. Furthermore, the thinner the constraining green layer, the weaker the constraining force for constraining the shrinkage. Consequently, the base green layer is more likely to shrink.
Accordingly, in a green laminate including a plurality of types of base green layers having different thicknesses in the range of, for example, 25 xcexcm to 300 xcexcm, in the case in which constraining green layers having the same characteristic are formed so as to be in contact with the main surfaces of the base green layers, the shrinking rates thereof may vary in the lamination direction of the laminate body when a monolithic ceramic substrate is formed by sintering the green laminate, and as a result, the laminate body may be warped. Furthermore, in a serious case, cracking and separation may occur in the laminate body. Consequently, the accuracy of positions at which the wiring conductors are provided for the laminate body is degraded, whereby formation of the wiring conductors having finer wiring density is prevented, and hence, reliability of the monolithic ceramic substrate thus obtained is decreased.
In the description of the related art above, the difference in thickness of the base green layers is exemplarily described as a factor which causes differences in the shrinking rates of the green layers. However, in addition the difference in thickness, the shrinking rates of the base green layers may differ from each other due to the difference in composition or type of material constituting the base green layers, the difference in wiring density distribution or distribution of the wiring conductors provided for the base green layers, or other factors.
In order to overcome the problems described above, preferred embodiments of the present invention provide a greatly improved monolithic ceramic substrate, manufacturing and designing methods therefor, and an electronic device including the novel monolithic ceramic substrate described above.
According to a preferred embodiment of the present invention, a monolithic ceramic substrate formed by sintering a green laminate includes a plurality of base ceramic layers which contain a low-temperature sinterable ceramic material and which are laminated with each other, a plurality of constraining layers which contain inorganic particles not sintered at a sintering temperature of the low-temperature sinterable ceramic material and which are each disposed so as to be in contact with the main surface of a predetermined layer of the plurality of base ceramic layers, the inorganic particles being bonded by diffusion of a part of the low-temperature sinterable ceramic material contained in the base ceramic layer adjacent to the constraining layer, and wiring conductors provided for the base ceramic layers.
In the monolithic ceramic substrate described above, in order to solve the problems described above, at least two constraining layers selected from the plurality of constraining layers have different constraining forces which are applied to base green layers for defining the base ceramic layers to constrain the shrinkage thereof during a sintering step.
In the monolithic ceramic substrate described above, it is preferable that the base ceramic layers include a relatively thick first base ceramic layer and a relatively thin second base ceramic layer, and the constraining layers include a first constraining layer disposed so as to be in contact with the main surface of the first base ceramic layer and a second constraining layer disposed so as to be in contact with the main surface of the second base layer, in which the thickness of the first constraining layer is larger than that of the second constraining layer.
In the monolithic ceramic substrate including a first and second base ceramic layer and a first and second constraining layer, described above, the particle diameter of the inorganic particles contained in the first constraining layer is preferably smaller than that of the inorganic particles contained in the second constraining layer.
In the monolithic ceramic substrate described above, it is preferable that the base ceramic layers include a first and second base ceramic layer having different thicknesses, and the constraining layers include a first and second constraining layer disposed so as to be in contact with the main surfaces of the first and the second base ceramic layers, respectively, in which the types of the inorganic particles contained in the first and the second constraining layers differ from each other.
In the monolithic ceramic substrate of various preferred embodiments of the present invention, the wiring conductors are preferably formed of a conductive material primarily composed of at least one metal selected from the group consisting of Ag, Au, Cu, Ni, Agxe2x80x94Pd, and Agxe2x80x94Pt.
In addition, in the monolithic ceramic substrate described above, the wiring conductors may have various shapes, and for example, the wiring conductors preferably include a conductive film extending along the main surface of the base ceramic layer and a via hole conductor extending so as to penetrate the base ceramic layer.
According to another preferred embodiment of the present invention, a method for manufacturing a monolithic ceramic substrate includes the steps of forming a green laminate including a plurality of base green layers which contain low-temperature sinterable ceramic particles and which are laminated with each other, a plurality of constraining green layers which contain inorganic particles not sintered at a sintering temperature of the low-temperature sinterable ceramic particles and which are each disposed so as to be in contact with the main surface of a predetermined layer of the plurality of base ceramic layers, and wiring conductors provided for the base green layers, and sintering the green laminate under conditions causing sintering of the low-temperature sinterable ceramic material, wherein at least two base green layers selected from the plurality of base green layers constituting the green laminate have different intrinsic shrinkabilities during the sintering step.
In the method for manufacturing a monolithic ceramic substrate described above, in order to solve the technical problems described above, that is, in order to constrain warping of the laminate caused by the difference between the shrinkabilities during the sintering step, at least two constraining green layers selected from the plurality of constraining green layers constituting the green laminate preferably have different constraining forces applied to the base green layers so as to constrain the shrinkage thereof.
In the method for manufacturing a monolithic ceramic substrate described above, the constraining force of the constraining green layer may be controlled by a factor of, for example, the thickness of the constraining green layer, the particle diameter, the type, the shape, the particle distribution, the content of the inorganic particles contained in the constraining green layer, and the surface condition of the constraining green layer, or may be controlled by a combination thereof.
In the method for manufacturing a monolithic ceramic substrate described above, the first forming step may further include a step of preparing base green sheets for forming the base green layers and a second forming step of forming the constraining green layers on the base green sheets.
In the method for manufacturing a monolithic ceramic substrate described above, the second forming step may further include a step of preparing a slurry to be used for forming the constraining green layers and a step of coating the slurry on the base green sheets, or may further include a step of preparing constraining green sheets for forming the constraining green layers and a step of overlaying the constraining green sheets on the base green sheets.
In the method for manufacturing a monolithic ceramic substrate described above, instead of the steps described above, the first forming step may further include the steps of preparing a base slurry to be used for forming the base green layers, preparing a constraining slurry to be used for forming the constraining green layers, coating the base slurry to form the base green layers, and coating the constraining slurry on the base green layers to form the constraining green layers.
Preferred embodiments of the present invention can also be applied to monolithic ceramic substrates manufactured by the methods described above.
In addition, preferred embodiments of the present invention can also be applied to a method for designing a monolithic ceramic substrate manufactured by the methods described above.
According to yet another preferred embodiment of the present invention, a method for designing a monolithic ceramic substrate includes a first step of sintering a composite formed by laminating a first test green layer containing the low-temperature sinterable ceramic particles and a second test green layer containing the inorganic particles under conditions causing sintering of the low-temperature sinterable ceramic particles to measure the shrinking rate of the first test green layer in the direction along the main surface thereof. In addition, this first step is performed for combinations of a plurality of types of first test green layers having different shrinkabilities from each other during sintering and a plurality of types of second test green layers having different constraining forces from each other during sintering, whereby the shrinking rates of the individual combinations are preliminarily obtained.
In addition, the method for designing a monolithic ceramic substrate described above preferably includes a second step of selecting a plurality of types of first test green layers having characteristics that are substantially equivalent to those of a plurality of base green layers required for forming a monolithic ceramic substrate, a third step of selecting some of the combinations of the first test green layers and the second test green layers, having the shrinking rates approximately equivalent to those of the plurality of selected types of first test green layers, and a fourth step of determining the characteristics of constraining green layers which are substantially equivalent to those of the second test green layers of the selected combinations.
The second step described above may have various modifications in accordance with factors in determining characteristics for selecting the first test green layers.
That is, when the thickness of the first test green layer is the factor, first test green layers may be selected having thicknesses that are substantially equivalent to those of the base green layers.
In addition, when the composition of the first test green layer is the factor, first test green layers may be selected to have compositions that are substantially equivalent to those of the base green layers.
Furthermore, when the wiring conductor provided on the first test green layer is the factor, first test green layers may be selected to have wiring conductors that are substantially equivalent to those provided on the base green layers.
In the method for designing a monolithic ceramic substrate according to preferred embodiments of the present invention, the fourth step described above may have various modifications in accordance with characteristics of constraining green layers to be determined.
That is, when the thickness of the constraining green layer is the characteristic to be determined, constraining green layers may be have thicknesses that are substantially equivalent to those of the second test green layers.
In addition, when the particle diameter of the inorganic particles contained in the constraining green layer is the characteristic to be determined, constraining green layers preferably contain inorganic particles having particle diameters that are substantially equivalent to those of the inorganic particles contained in the second test green layers.
Furthermore, when the type of inorganic particle contained in the constraining green layer is the characteristic to be determined, constraining green layers preferably contain inorganic particles that are substantially equivalent to the types of inorganic particles contained in the second test green layers.
Preferred embodiments of the present invention can also be applied to an electronic device including the monolithic ceramic substrate described above and a motherboard having the monolithic ceramic substrate mounted thereon.